Leading City shareholders piled the pressure on AstraZeneca by calling on the drugs group’s directors to justify why they flatly rejected a £63 billion offer from Pfizer, shunning merger talks with their American suitor.

Britain’s second largest pharmaceuticals company snubbed a sweetened £50-a-share proposal from Pfizer after a brief board meeting yesterday.

Pascal Soriot, chief executive, told The Times: “There was no debate. There was a very quick conclusion by everybody that the offer wasn’t one we could entertain.”

The stand-off raises the possibility that Pfizer could launch a hostile bid, despite legal obstacles. Experts said that if a takeover